Abstract An Epigenetic Axis in Bone Formation As over 53 million adults in the United States have low bone mass due to arthritis and osteoporosis, novel therapeutic strategies that protect from progressive and chronic bone loss are clearly warranted. The purpose of the proposed research is to refine our central hypothesis, based on my previous research, the miR-23a cluster controls bone synthesis and homeostasis by regulating Runx2-dependent Ezh2 expression. Current studies do not have any insight on how epigenetic repression by histone methylation of bone specific chromatin is maintaining bone mass in vivo. Understanding of this gap is pivotal for further studies to identify specific targets for future therapies, which protect against bone loss. In our previous work we found that miR-23a cluster represses Runx2 expression, inhibits osteoblast specific genes, and finally blocks osteoblast differentiation in vitro. However, the inhibitory role of miR-23a cluster during the development and remodeling of bone and the chromatin repression mechanism, leading to this inhibition of essential genes required for bone formation, is unrevealed. To address the above queries in vivo for this bone-regulating miRNA cluster, we created an inducible anti-miR-23a cluster (miR-23aClZIP) knockdown mouse model. Interestingly, our miR-23a cluster knockdown mice developed high cortical and trabecular bone. RNA sequencing from these mice displayed an increased expression of Runx2, the master transcription factor essential for skeletogenesis, and decreased expression of Ezh2, a chromatin repressor also vital for skeletogenesis. Furthermore, we observed that Runx2 decreased the repressive activity of Ezh2 to the promoters of bone essential genes for osteogenesis. Together, our findings strongly suggest that miR-23a cluster connection with a tissue specific RUNX2?EZH2 function is a very unique regulatory mechanism. Thus, this mechanism will be key to the genetic basis of bone development, growth and maintenance further. However, additional research is needed to clarify 1) bone phenotype of miR- 23aClZIP mouse during development and maintenance; 2) compositions of the EZH2 led PRC2 methylation complex and their recruitment; finally, 3) histone modifications and mechanisms that led to enhanced bone formation. We will refine our central hypothesis by testing two hypotheses derived from the central hypothesis and stated in the Specific Aims below. Together these specific aims will address the unresolved issues outlined above. Specific Aim 1. In vivo inhibition of miR-23a cluster enhances bone synthesis and remodeling. Specific Aim 2. In vivo inhibition of miR-23a cluster decreases EZH2 controlled methylation and modifications of bone gene promoters. Exploring our novel mouse model with cutting edge in vivo and in vitro molecular tools, we will accomplish the above specific aims, and later translate these results into osteoporotic and osteopenic bone loss models, to block this repressive state of chromatin and protect from such bone loss. This is how our current study will remove a barrier to progress the bone field.